The present invention relates to a coupling element for coupling a source of elliptically-shaped radiation such as a laser diode to a conventional circularly symmetric single-mode optical fiber, and to a method of making such element.
The coupling efficiency between laser diodes and conventional single-mode fibers is affected by the aspect ratio of the radiating area of the diodes. The elliptically shaped laser mode must be transformed into an appropriately sized, circularly symmetric fiber mode.
A circularly symmetric lensed fiber that is situated several microns in front of the laser facet can be employed if the light beam is fairly round, i.e. if its aspect ratio is no greater than 2:1. One such scheme has been reported to provide a coupling efficiency of 90%. However, typical laser diodes are much more elliptical (aspect ratio of about 3.5:1) for which the maximum coupling efficiency to a circularly symmetric lensed fiber is about 65-70%.
A more efficient coupling between a laser diode and a circularly symmetric single-mode fiber has been effected by heating the end of the fiber and then flattening it to provide the desired aspect ratio. Such a method is disclosed in U.S. Pat. Nos. 4,370,021 and 4,763,975. Because of viscosities of the fiber core and cladding glass during flattening step, core glass emerges from the fiber endface in form of semi-ellipsoidal lens. The shape of a lens that is formed by flattening the end of the fiber is not easily controllable; it is usually too steep to provide the desired focusing properties.
In a modification of the above-described method, the endface of the flattened fiber end portion is presumably ground flat, since a reflecting layer that forms part of the laser is deposited on the end of the fiber core. Thereafter, a lens is added to the fiber endface by applying thick layer of photoresist on the elongated endface which is then exposed via the fiber core and developed. The curvature of the resultant semi-ellipsoidal lens cannot be shaped independently of the aspect ratio.